It's important to note that different cultures have different predominant ideas of what constitutes the greatest horror. The most popular trend in Japanese horror, for example, is horror that cannot be understood, whereas in much of American horror, it is invincible horror, horror that cannot be destroyed. These two have some relation since horror that cannot be understood probably also cannot be fought effectively, but it's a different approach in most cases. The difference in the end goal means a very different approach.
A common belief of horror is that revulsion is crucial in the genre. A feeling of revulsion sets a strong foundation for a chronicle of horror. At the same time, it can be said too that a story which tempts its viewer or reader is also effective in horror, as it can tempt the reader into continuing to read and then deliver an impact that terrifies them, which due to their willing investment, they continue to read.
Personally, when I want to present something that I see as horror, I present it as something I personally find unsettling. Since I'm a weird person already, I figure that something strange to me must be insane to most people! I agree that it's good not to overexplain the horror. Leave it a little mystery to its nature. The most effective monsters are frightening and mysterious; they cannot be understood, because the moment you quantify them, they become less formidable. If you can understand why a killer kills, that killer is no longer frightening but tends to become pitiable, which is the last thing you want for a horror figure.
It's why, in John Carpenter's Halloween, Michael Myers as an adult is known as The Shape. There is no reason for him to kill, not really. He does it like a machine, and there is nothing human in him. To quote Dr. Loomis from that same film, "I met him fifteen years ago. I was told there was nothing left. No reason, no, uh, conscience, no understanding and even the most rudimentary sense of life or death, of good or evil, right or wrong. I met this six year old child with this blank, pale, emotionless face, and the blackest eyes, the devil's eyes. I spent eight years trying to reach him and then another seven trying to keep him locked up because I realized that what was living behind that boy's eyes was purely and simply...evil." He has become only a humanoid shape propelled by what can only be called pure evil.
In visual terms, adding excessive detail and "realing it up" can actually make something more horrific to most audiences. It's one of the reasons why the development of a style is essential to an artist, because if that artist simply makes something excessively detailed, it will cultivate a sense of revulsion and, through it, horror to most audiences. It's especially well done by mangaka like Itou Junji, and you'll probably notice the excess of details on the images meant to be especially impactful.
I hope this will help you! 
